1. Field of the Invention
The present invention relates to a rare gas fluorescent lamp lighting apparatus, and more particularly to a rare gas fluorescent lighting apparatus to light a rare gas fluorescent lamp as a light source in a document reading device, such as a facsimile machine, an image scanner, and a copying machine.
2. Description of the Related Art
Recently, a rare gas fluorescent lamp, which emits light through rare gas discharging, is increasingly used as a light source for illumination in a document reading device, such as a copying machine, and an image scanner. It is well known that a rare gas fluorescent lamp is lighted with a high-intensity luminance when a high-frequency pulse voltage is applied, and a lighting apparatus with an inverter circuit to generate such a high-frequency pulse voltage is preferably employed for lighting a rare gas fluorescent lamp.
On the other hand, there is a problem with a rare gas fluorescent lamp that when a lamp current is varied due to the fluctuation of an input voltage, or the like during the operation of the aforementioned document reading device, the light amount of a lamp is varied thereby deteriorating accuracy of reading a document, which results in impairing image reproduction quality. In order to deal with this problem, a rare gas fluorescent lamp lighting apparatus is proposed, in which a DC-DC converter is put at the input side of the circuit to generate a high-frequency voltage thereby stabilizing electric power at the input side so that the light mount of a rare gas fluorescent lamp is stabilized without suffering the influence of the fluctuation of the input voltage (refer to, for example, Japanese Patent Application Laid-Open No. 2001-15284).
FIG. 18 is a block diagram of such a rare gas fluorescent lamp lighting apparatus as disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2001-15284, in which a step-up DC-DC converter CV is provided at the input side of a high-frequency voltage generating circuit HC. A second driving circuit CT in the DC-DC converter CV is adapted to feed a PWM modulated driving signal to a second switching element S2 based on a signal fed back from a current detecting circuit R so that a current detected at the current detecting circuit R has a constant peak value, whereby the output is controlled at a constant electric power without receiving the influence of voltage fluctuation of a DC power supply E1 thus stabilizing the light amount of a rare gas fluorescent lamp DL.
The rare gas fluorescent lamp lighting apparatus shown in FIG. 18, which includes the step-up DC-DC converter CV disposed at the input side of the high-frequency voltage generating circuit HC as described above, suffers deterioration in efficiency due to the switching loss of the switching element S2, the copper loss and the iron loss of a coil L1, and the loss at a diode D1, and also requires a large space for mounting components thus preventing downsizing of the apparatus. And, the second driving circuit CT is usually constituted as an IC, and therefore the entire cost is pushed up.